A “smart lock” replaces (or supplements) your deadbolt and allows you to
open your door via a Bluetooth connection. Nearly all of them let you
lock or unlock your door from an iOS or Android phone, and some of them
also have keyfobs that you can use as well. Most of them have an “autolock”
feature that will automatically lock your door after a period of time.
Some of them will automatically open the door if you’re within range
(and they claim to be able to tell if you’re on the outside so that the
door doesn’t automatically unlock when there’s an ax-wielding murderer
on the outside and you approach the door with your phone in your pocket
from the inside). Most of them will notify you (or keep a log) of comings
and goings so that you can track activity through your front door.

Some of them allow you to give out “electronic keys” to visitors, and
can even time-limit them. For example, you could give a key to a plumber
that would allow him or her free access between 1-4PM, but lock them
out otherwise. Or, if you have a house guest, you could send them an
electronic key that would let them arrive during the day when you’re not
around.

The great thing about these features is that most of them are implemented
in software, so you could potentially expect new features to be added
even after you’ve installed the device.

Danalock ($179) is a Scandinavian device that’s not too
well-known in the USA, but they appear to have set the standard. It’s a
battery-powered cylinder that installs on the inside of the door, replacing
the normal deadbolt handle. In addition to Bluetooth, it supports Z-wave,
which gives you access over the Internet if you have a compatible home
automation hub. The reviews on Amazon are generally unenthusiastic, with
problems ranging from lag (i.e., it takes a bit of time from when you
command it to unlock to when it actually unlocks) to incompatibility with
various door hardware. Some reviews mention “likely require sawing off pieces
from your existing lock” which is scary.

August ($249) is the newer, cooler, American big-brother
to Danalock. It uses heavier batteries than the Danalock, and I’m guessing
that it has a more powerful motor, since there are fewer complaints about
that sort of thing. It’s Bluetooth only, which means that you can’t control it
remotely unless you get the additional August Connect ($49) which is a
WiFi-Bluetooth bridge. The scariest thing in the reviews is that people mention
that it will sometimes unlock itself, randomly, in the middle of the night.
Not exactly something to inspire confidence. August/August Connect will
work with Nest which is IMHO a big advantage since it
can, for example, turn your thermostat back on when you arrive from being out.
August Connect will only control a single lock, so you have to have both if
you want to control them remotely.

One of the nice things about both the Danalock and the August is that it
merely replaces the deadbolt handle. Thus, your keys will still continue
to function. If you have multiple doors in your house with the same key,
you don’t have to replace all of them (or deal with having different keys
for different doors).

The Kwikset Kevo ($199) takes a different approach,
in that it replaces the entire deadbolt assembly. It is Bluetooth only
and, as far as I can tell, there’s no way to connect it to the Internet.
It’s controlled from your smartphone and lets you send “e-keys” to people,
though it charges for them. A big advantage of the Kevo seems to be that
they kept the deadbolt handle on the inside, so you can see at a glance
if the door is locked or unlocked. The Kevo will also work with a
plastic keyfob that you can give to a child or a friend, but additional
ones cost $25 each.

Updates

2015-11-23: The
Schlage Sense
is a new offering in the smart lock space. It has the option of allowing
entry via a numeric keypad (you can generate unique codes for
visitors so that you can track who is coming and going) and also
permits Bluetooth automatic unlocking. As a bonus, if you have a
Bluetooth-enabled Apple TV near the lock, you can control it
remotely.

2018-01-08: I ended up installing the Schlage Sense a few years ago.
For the most part, it’s a very decent lock.
However, it has the problem of relying upon Apple’s HomeKit, which meant that
(a) you had to have the lock within range of an Apple TV, and
(b) it wouldn’t work with the Amazon Echo.
Now, the Amazon Echo didn’t really exist when I started this effort,
so I made a bad bet on the future of HomeKit.
I ended up with dozens of smart-home devices that could be
controlled via the Echo/Alexa, and only one that required HomeKit
(the front door lock).

Schlage recognized the problem and released an add-on WiFi adapter
for the Sense lock. This converts the device into a standalone
item that can be controlled directly or via the Echo/Alexa
without requiring HomeKit. I installed one of these a few days
ago, and it seems to work great, except that it no longer
works with “Hey, Siri.”